Third Generation Partnership Project (3GPP) Long Term Evolution (LTE) technology is a mobile broadband wireless communication technology in which transmissions from base stations (referred to as eNBs or as cell nodes) to mobile stations (referred to as user equipment (UE)) are sent using orthogonal frequency division multiplexing (OFDM). OFDM splits the signal into multiple parallel subcarriers in frequency. The basic unit of transmission in LTE is a resource block (RB) which, in its most common configuration, consists of twelve subcarriers and seven OFDM symbols (one slot). A unit of one subcarrier and one OFDM symbol is referred to as a resource element (RE) 1, as shown in FIG. 1. Thus, an RB consists of eighty-four REs. An LTE radio subframe 2 is composed of two slots in time and multiple resource blocks in frequency with the number of RBs determining the bandwidth of the system, as shown in FIG. 2. Furthermore, the two RBs in a subframe that are adjacent in time are denoted as an RB pair. Currently, LTE supports standard bandwidth sizes of 6, 15, 25, 50, 75 and 100 RB pairs.
In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 ms.
The signal transmitted by the eNB in a downlink (the link carrying transmissions from the eNB to the UE) subframe may be transmitted from multiple antennas and the signal may be received at a UE that has multiple antennas. The radio channel distorts the transmitted signals from the multiple antenna ports. In order to demodulate any transmissions on the downlink, a UE relies on reference symbols (RS) that are transmitted on the downlink. These reference symbols and their position in the time-frequency grid are known to the UE and hence, can be used to determine channel estimates by measuring the effect of the radio channel on these symbols. In Rel-11 and prior releases of LTE, there are multiple types of reference symbols. The common reference symbols (CRS) are used for channel estimation during demodulation of control and data messages in addition to synchronization. The CRS occur once every subframe.
Heterogeneous networks, where the macro cells and the small cells have vastly different transmit powers, may be deployed in two main ways. In the first deployment type, the small cell layer and the macro cell layer share the same carrier frequencies which creates interference between the two layers. In the second deployment type, the small cell layer and macro cell layer are on separate frequencies. The network architecture for LTE allows messages to be sent between eNBs via an X2 interface. The eNB also can communicate with other nodes in the network, e.g., to the Mobility Management Entity (MME) via the S1 interface.
The existing specifications for home eNBs (HeNBs) allow some self-organizing network (SON) functionality, where an eNB can request information regarding another eNB via the MME. In FIG. 3, the architecture 3, involving evolved universal terrestrial access network (E-UTRAN), the radio access network (RAN) and the core network (CN) is shown. According to current specifications it is possible for an eNB 4 to request SON information via an S1 procedure called eNB Configuration Transfer. Within a CONFIGURATION TRANSFER message from the eNB 4 to the MME 6, it is possible to indicate a target eNB identification (ID) and the SON information that are required from that target eNB 8. The MME 6 will forward such an information request to the target eNB 8 via a procedure called MME Configuration Transfer. Once the target eNB 8 receives the request it will reply via the eNB Configuration Transfer procedure towards the MME 6. The reply will include the information requested by the source eNB 4. The MME 6 will forward the information requested to the source eNB by means of a new MME Information Transfer.
If a source eNB 4 requests time synchronization information from a target eNB 8, the reply contained in the SON Configuration Transfer IE from target eNB 8 to source eNB 4 should include the following elements:
1) Stratum level: This is the number of hops between the eNB and the synchronization source. That is, when the stratum level is M, the eNB is synchronized to an eNB whose stratum level is M−1, which in turn is synchronized to an eNB with stratum level M−2 and so on. The eNB with stratum level 0 is the synchronization source.
2) Synchronization status: This is a flag that indicates whether an eNB is currently in a synchronous or asynchronous state.
Many features of 3GPP Long Term Evolution (LTE) technology benefit from the base stations (referred to as eNBs) in the system being synchronized with each other with respect to transmit timing and frequency. Synchronization of eNBs is typically done using a global navigation satellite system (GNSS) such as the Global Positioning System (GPS) or by using network based methods such as IEEE 1588v2. However, when such methods are unavailable to an eNB, it may be possible to use LTE reference signals transmitted by other eNBs to acquire synchronization. Such techniques have been discussed in 3GPP for small cells in LTE Rel-12 where a small cell can obtain synchronization from a macro cell or from other small cells.
One of the scenarios being considered is the case where the macro cell layer is not synchronized, i.e., the timing of the macro cells is not aligned, whereas the small cell layer in the coverage of the macro layer is desired to be synchronized. When small cells in a cluster of cells that are on the border of two macro cells use a macro eNB to obtain synchronization, it may be possible that the small cells in the cluster synchronize to different macro eNBs. When this happens, small cells in the same cluster may not be time-synchronized with each other since the different macro eNBs that were used as the synchronization source are not synchronized between themselves.
One of the existing solutions to this problem is to limit the number of small cells in a cluster that can synchronize to a macro cell to one and then to let all the other small cells synchronize to this master cell either directly or via other small cells that have synchronized to this master cell. This can ensure that cells in the small cell cluster do not synchronize to different macro cells. However, the problem with this solution is that an eNB may be separated in the synchronization hierarchy from the master eNB in the cluster by many hops. For example, an eNB may synchronize to another eNB which has in turn synchronized to another eNB which has synchronized to the master eNB. The number of hops in this case is three. When the number of hops increases, the estimation errors in each hop may accumulate so that the synchronization accuracy for the eNB with a high hop number may be compromised.
Another solution is to ensure that every eNB has a GNSS receiver and/or is connected to a backhaul that can support network based synchronization. This solution can, however, lead to greater cost and also may not be feasible in some cases where satellite coverage or a good backhaul connection is not possible.